Temperature-modulated differential scanning calorimetry - IX. Some comments on the rigid amorphous fraction in semi-crystalline poly(ethylene terephthalate)

The increment of heat capacity at the glass transition for semi-crystalline poly(ethylene terephthalate) (PET) observed by temperature-modulated diffe rential scanning calorimetry (TMDSC) shows significant deviations from a si mple crystalline/amorphous two-phase model. Introduction of a rigid amorpho us fraction, which is non-crystalline but which also does not participate i n the normal glass transition, allows a much better description of the tran sition behaviour in semi-crystalline PET. Certain questions arise such as w hat is the rigid amorphous fraction and over what temperature range do thes e rigid amorphous segments devitrify? These TMDSC results show that the rig id amorphous component may be treated as an interphase between amorphous an d crystalline phases. This interphase does not exhibit a separate glass tra nsition temperature at temperatures above the normal T-g. The suggestion is made that the glass transition of the rigid amorphous component occurs con tinually between the glass transition temperature of the amorphous phase an d up to about 135 degrees C for this particular sample of PET.